s=input()
mx=0
if len(s)==4:
    a=[s[0], s[1],s[2], s[3],s[0:2], s[1:3],s[2:4], s[0:3], s[1:4], s]
    for j in range(len(a)):
        n=int(a[j])
        c=0
        for  i in range(1, n+1):
            if n%i==0:
                c+=1
        if c==2:
            mx=max(mx, n)
print(mx)
